X inactivation and Dosage Compensation Flashcards
How do human females account for the difference in dosage of X specific DNA to males
Females have one X inactivated in every cell, this process is called lyonisation, the inactive X is seen in the nucleus as a barr body. In examples where there are more than 2 X then all X are inactivated excpet one. The choice of this x is random at differentiation and then clonally maintained.
What are the features of the inactive X chromosomes and what structure is required for X inactivation to occur?
Barr body is heterochromatised, late replicating and associated with the nuclear envelope. The histone 4s are hypoacetylated and the promotors of genes are hypermethylated.
For inactivation to occur there must be two chromsomes containing the X inactivation center to allow counting to take place. This was discovered via translocation experiments.
What stimulates the inactivation in humans?
A gene on the XIC called XiST (specific transcripts) produces an untranslated RNA from both chromsomes, the transcript becomes stable in one chromosome and not the other resulting in transcription being switched off for XiST in Xa. In mice this XiST is present before inactivation so we know it is causal and KO prevents inactivation from occurring.
Humans with mutations in the XiST have non random inactivation and moving an XiST onto an autosome in mice results in gene inactivation.
How is transcription from Xa stopped?
Tsix is an antisense regulator of XiST. It is transcribed from the opposite strand and alters the chromatin structure of XiST promoter on the active X to prevent XiST transcription.
If this antisense strand is mutated then there is preferential inactivation of that X.
What is the mouse model of X inactivation?
Pre inactivation there is a low transcription of XiST due to Tsix preventing induction of transcription. As such there is no cis association.
Cell differnetiation triggers inactivation and counting is thought to be due to transient pairing of the XIC - unclear how a decision is made.
Execution - Tsix is downregulated on Xi and upregulation of Tsix and there is stabilisation of XiST on Xi. XiST associated with Xi chromosome via way stations (May be L1s - Human X has high density). XiST coats Xi and recruit polycomb complexes, chromatin modifications and gene silencing by methylation.
Maintenance of Xi does not require XiST but many methylation pathways are implicated.
Are still many genes that are required in the XIC but with unknown function.
How are humans different to mice?
Human ES cells undergo X inactivation soo early it is difficult to study.
Different genes are implicated. Tsix is a pseudogene and different genes seen in human XIC. RNF12 (thought to be counter) is outside of XIC.
Tsix expressed from inactive X for a long time and can’t repress XiST - mystery. Humans appear to have selection of Xa and no inactivation of all Xi.
Why might X genes be upregulated to double dose?
On the original proto X, some genes may have interacted with autosomal genes in a dosage sensitive manner e.g. dimer TF.
This would need to be maintained and so some X genes may be up-regulated. Also as homologous genes were lost on Y this would become increasingly important.
Are Xa and the male X chromosome upregulated?
Yes genome wide expression analysis in HapMap samples showed that the Xa and X in men express their single genes the same amount as 2 normal autosomes.
What issue does the up regulation of Xa and the male X cause?
There must be some genes escaping inactivation because the Y chromosome contains some homologous genes to the X i.e. PAR1 and PAR2.
Which X linked genes on Xi escape inactivation?
PAR1 genes escape inactivation on Xi. Other genes also have some expression but this is probably due to them not being maintained well or not being dosage sensitive.
PAR2 is unusual - 2/4 are inactive on Xi (but not by the normal method) and inactive on Y - probably due to proximity to Y heterochromatin this is called zone 1. The other 2 escape inactivation on Xi and are expressed normally on the Y this is zone 2.
Zone 1 has 35% G-C rich and Zone 2 is 50% G-C rich. This G-C richness for zone 2 is similar to PAR2.
Inactivation of zone 1 genes occurs differently for each. One uses methylation other uses other epigenetic mechanisms.
How do aneuploidies differ between sex chromsomes and autosomes?
13, 18 and 21. All have low gene count as the phenotype that arises are as a result of gene dosage.
Sex aneuploidies are fairly common. Y not required for life and any extra X’s are inactivated.
What is Turner’s and why does it present problems?
Turner’s syndrome = 45X. It has a very high in utero mortality. Majority of the time it occurs as a result of an X from the mother and non dis junction in the father. Can also occur as a result of X p-arm deletions from the father.
Presence of disease suggests that there are other homologous genes on the Y that escape inactivation on the Xi. Prime candidates would be PAR1 and PAR2 regions. The SHOX gene in PAR1 is the only known disease locus in the PAR regions where copy number changes can result in shorter long bones - also seen in Turner’s but doesn’t explain the whole phenotype.
What could be causing the phenotype in Turner’s sydnrome?
Some regions of the Y have very similar sequences other than PAR1 and PAR2 reflecting the history of the Y. These genes are called gametologs candidates for Turner’s include RPS3X/Y - involved in ribosomal protein subunit. Many other gametologs suggested.
Fact that majority of Turner’s die in utero may hold key. Inactivation occurs at the blastocyst stage. Perhaps double dosage of X specific genes are required prior to inactivation for female development.
What happens when there are extra X chromosomes?
Klienfelter’s syndrome - 47XXY - live male births, small testes, azoospermia and gynaecomastia
47XXX - mostly unaffected but some reduction in IQ, 25% are infertile. 10-15pt decrease in IQ per extra chromosome.
47XYY - rare but fairly normal. 48XYYY and 49XYYYY rare with behavioural and developmental problems.
